This invention relates to a method of producing a glass optical element with a high precision and to a glass substance processed by the method into the glass optical element.
A wide variety of development and research have been made about the technique of press-forming or press- molding a glass substance within a mold to produce a glass optical element of high precision. For example, one attempt has been directed to a mold which has a molding surface of silicon carbide, silicon nitride, and the like.
Herein, silicon carbide and silicon nitride are excellent in hardness and strength against a high temperature. Such a molding surface of silicon carbide and/or silicon nitride may be deposited by a chemical vapor deposition (CVD) method. In this event, the molding surface has an excellent compactness without occurrence of surface defects, such as pores, and can be polished into a mirror surface. However, the molding surface of such materials is liable to be oxidized to form a silicon oxide surface layer of several tens of angstroms thick. In this case, it has been pointed out that fusion sticking often takes place between the molding surface and the glass substance during a press-forming step when the glass substance is composed of borosilicate glass or silicate glass containing a large amount of modification components, such as alkali or alkaline earth positive ions. Moreover, stress is concentrated in a following cooling step here and there on the molding surface, which causes cracks to occur on the molding surface. This results in a phenomenon such that the molding surface of the mold is scooped or removed in spots. This phenomenon will hereinafter be simply called a "pullout" or a "pullout" phenomenon.
Taking the above into consideration, Japanese Patent Publication (B4) No. 62816/1992 and Japanese Unexamined Patent Publication (A2) No. 199036/1990 disclose a method of forming a carbon thin film on the molding surface of silicon carbide or silicon nitride. The carbon thin film may be either a hard carbon film or an i-carbon film and serves to prevent the above-mentioned fusion sticking of the glass substance to the mold. By coating the molding surface of silicon carbide or silicon nitride with the carbon thin film, the fusion sticking and the pullout can effectively be avoided. Thus, the carbon thin film is helpful to release from the molding surface the glass substance pressed by the mold and may be called a releasing thin film.
However, it is impossible to form a perfect carbon thin film which is free from defects and which completely covers an entire area of the molding surface of the mold. In other words, film defects are microscopically observed in the carbon thin film. This fact is disclosed in Japanese Unexamined Patent Publication (A2) No. 120245/1990 and well known in the art.
If the carbon thin film has such film defects, silicon carbide or silicon nitride of the molding surface is exposed through the film defects of the carbon thin film and locally oxidized to form the silicon oxide surface layer. Such local oxidization of the molding surface causes the pullout to occur due to both the fusion sticking of the glass substance to the mold and the stress concentration during repetition of the press-forming step and the cooling step.
In addition, the carbon thin film is not permanently invariable and may be peeled off from the mold by repetition of the press-forming step followed by the cooling step as a result of oxidization of silicon carbide or silicon nitride, which weakens the adhesion strength of the carbon thin film.
Taking the above into consideration, the carbon thin film is forcibly detached and removed from the molding surface after these steps are repeated over a predetermined period. Thereafter, a new carbon thin film is formed to reproduce or reuse the mold.
However, the film defects inevitably take place in the new carbon thin film. If the press-forming step is repeated by the use of the mold with the film defects left in the carbon thin film, the pullout is caused to occur as described in the foregoing. The spread of the pullout not only results in degradation of a glass optical element but also in an unrecoverable damage of the mold.
As described above, it is technically impossible in an industrial scale to form a non-defect carbon thin film and also to completely cover the entire area of the molding surface of the mold with such non-defect carbon thin film.
Under the circumstances, consideration might be directed to the glass substance to be processed by press-forming. However, no disclosure has been made yet about an effective glass substance.
Recently, Japanese Unexamined Patent Publication (A2) No. 345461/1994 proposes a glass substance which includes neither arsenic oxide nor antimony oxide and which can be readily press-formed. Specifically, this proposal is based on the following facts. Namely, a dense crown glass is exemplified as a glass substance which contains by weight 0.2% arsenic oxide and 0.2% antimony oxide as a refining agent and a decoloring agent. The glass substance is placed in a mold which has a molding surface coated with an amorphous diamond-like carbon thin film and is heated to a temperature between 750.degree. C. and 1250.degree. C. In this event, reaction occurs between the carbon thin film and oxygen gas released from arsenic oxide and antimony oxide. As a consequence, the carbon thin film is oxidized, consumed and partly peeled off. In order to avoid such reaction, arsenic oxide and antimony oxide are excluded from the glass substance proposed in the above-mentioned publication.
However, exclusion of both arsenic oxide and antimony oxide from the glass substance brings not only about deterioration of a seed-free characteristic of a glass melt but also about decoloration of the resultant glass article. It is therefore required to solve these disadvantages.